1. Field of the Invention
The present invention relates to an image heating apparatus for heating a toner image on a recording material. Examples of the image heating apparatus are a fixing device for heating and fixing a non-fixed toner image on the recording material, and a glossiness enhancing device for heating the fixed toner image on the recording material to enhance glossiness of the image.
2. Description of the Related Art
Conventionally, an image fixing device disclosed in Japanese Utility Model Application Publication No. 64-6514 is known as an image heating apparatus for heating a toner image on a recording material such as a paper sheet in an image forming apparatus such as a printer. In the image fixing device, a heating roll in which a pair of sheet positive character thermistor elements (heater elements) is integrally incorporated in a cylindrical rotation roll is used as a heat source. Therefore, according to the image fixing device, when a recording material on which a toner image is transferred passes between the heating roll and a pressure roller which is in contact with the heating roller under pressure, the recording material is heated and pressurized, and the toner image is fixed.
As an image heating apparatus using a sheet-like heater (heater element) as a heat source, there is one disclosed in Japanese Patent Application Laid-open No. 5-226063. According to the technique disclosed in the Japanese Patent Application Laid-open No. 5-226063, in order to prevent one sheet-like heater from being locally excessively heated, the sheet-like heater is divided into six heaters, the six heaters are divided into two group every other one, and the two groups are alternately ON/OFF controlled.
If the image heating apparatus uses the many heaters as the heat sources, there is a possibility that abnormal states are encountered in the respective heaters. Hence, in the image heating apparatus using the many heaters as the heat sources, it can be conceived to detect an abnormal state utilizing a variation in temperature of the entire heaters.
Japanese Patent Application Publication No. 6-89901 discloses a technique for displaying abnormal states of respective heaters incorporated in an electric carpet. According to the technique disclosed in the Japanese Patent Application Publication No. 6-89901, thermal wires are provided in adjacent to three heaters having different heating regions, a temperature signal circuit and a thermal wire abnormality detecting circuit are connected to each thermal wire. Relay contacts are provided between the three heaters and power supply, and a relay abnormality detecting circuit is connected to each relay contact. If an abnormal state is detected from the temperature signal circuit, the thermal wire abnormality detecting circuit or the relay abnormality detecting circuit, each heater displays which portion of the corresponding heating regions (left surface, central surface and right surface) has the abnormal state.
In the case of the structure for detecting an abnormal state using the variation in temperature of all of many heaters, if one heater out of n heaters gets out of order, a variation amount of detection output caused by this trouble is 1/n. That is, as the number of heaters is increased, the variation amount of detection output caused by the abnormality is reduced, and it is difficult to secure a sufficient S/N ratio enough to detect abnormality. Thus, it is difficult to detect that a trouble is negated in a specific heater from the temperature variation of all of the many heaters.
According to the Japanese Patent Application Publication No. 6-89901, since the temperature detecting thermal wire is disposed in each of the heaters, it is easy to detect that a trouble is generated in particular one heater out of the many heaters, but there is a problem that the structure becomes complicated and the cost is increased.